Why refuse to call Passover, Passover then?
Because Pascha includes BOTH Passover and the Feast of Unleavened Bread.
"πάσχα páscha, pas'-khah; of Chaldee origin (compare
H6453); the Passover (the meal, the day, the festival or the special sacrifices connected with it):—Easter, Passover.
The paschal feast, the feast of the Passover, extending from the 14th to the 20th day of the month Nisan." "Passover" would mean only the 14th of Nisan but that is not what the narrative is about.
JOHN GILL'S COMMENTARY
Acts 12:4 And when he had apprehended him, he put him in prison, and delivered him to four quaternions of soldiers to keep him; intending after Easter to bring him forth to the people.
When he had apprehended him,....
When his officers he sent to take him had brought him: he put him in prison; in the common prison, very likely where he had been once before, Acts 5:18 and delivered him to four quaternions of soldiers to keep him; each quaternion consisted of four soldiers, so that they were in all sixteen; and so the Syriac version renders it, "and delivered him to sixteen soldiers": how the Ethiopic version should make "seventeen" of them is pretty strange: these perhaps might take their turns to watch him by four at a time, two to whom he was chained, and two others to keep the doors; or all the sixteen together, being posted in one place or another for greater security: and it may be, that the reason of all this caution, and strong guard, might be, because it was remembered that he, and the rest of the apostles, when committed to the same prison some years ago, were delivered out of it:
intending after Easter, or the passover, to bring him forth to the people; to insult and abuse him, and to put him to what death they should desire.
Acts 12:5 Peter therefore was kept in prison: but prayer was made without ceasing of the church unto God for him.
Peter therefore was kept in prison,.... Till the feast of the passover was at an end:
but prayer was made without ceasing of the church unto God for him; this was not done by them as a body together, but either by them in several bands at different places, or by some of the principal of the church at some one certain place, and where they might frequently change companies, and keep on a continual incessant prayer for days together; and whereas it is very likely it might be at the beginning of the passover, when Peter was taken up, and it was now at the close of it, when he was delivered, the church might be engaged by companies alternately, a whole week together, in prayer, on this occasion.
So in fact the whole Pascha week was involved, and "after Easter" would mean after the Sunday which was within that period. By the 17th century Easter was synonymous with the resurrection, and its pagan origins were meaningless.
Regardless, today even the unbelieving world understands that Christians celebrate the resurrection of Christ on this day.